Often apparatus of this kind have to be operated at locations where vibrations, such as acoustic vibrations, are present, like in production facilities for semiconductors, also known as ‘FAB’s. In such circumstances it is important to use enclosures to insulate the apparatus from its environment, to be able to operate these apparatus within their boundary conditions.
Consequently enclosures with a substantial rectangular configuration are known which are adapted to contain an apparatus sensitive for acoustic vibrations, the enclosure comprising walls and acoustic damping material located within the wall.
These prior art enclosures need to be voluminous and heavy to be able to effect a sufficient insulation. This appears from the thickness of the walls which is commonly between 50 mm and 100 mm. This thickness is however often insufficient to provide the desired acoustic insulation. Of course the enclosure could be built thicker, but this either leads to a smaller internal volume of the enclosure, leaving less space around the apparatus, which is awkward during the installation and servicing, or to a larger external volume of the enclosure, resulting in added use of floor space.
U.S. Pat. No. 4,362,222 discloses an enclosure with a substantial rectangular configuration, adapted to contain an apparatus sensitive to acoustic vibrations, the enclosure comprising walls and acoustic damping material located within the wall, wherein the acoustic damping material comprises at least one absorbing body of acoustic energy absorbing material having the shape of a parallelepiped located adjacent to an edge of the enclosure.
In this prior art structure the damping material his formed by slabs having only a limited thickness, coherent with that fact that only a limited damping of acoustic frequencies in the frequency range for which the human ear is sensible is aimed for.
It has appeared to the inventor that the acoustic vibrations most disturbing the processes in the apparatus within the enclosure are surprisingly caused by standing acoustic waves within the enclosure. In most cases these apparatus are particularly vulnerable for vibrations with frequencies in the range between 50 Hz and 1000 Hz, as caused by the nature of these apparatus. This frequency area of the vibrations to be avoided is rather different from the frequency area for which the human ear is in particular sensible. This discrepancy avoids that prior art insulating features known to be effective for protection of the human hearing can be simply adapted for this purpose.
Further DE-U-200 11 448 discloses a building wherein absorbing bodies are arranged suspended on horizontal lines allowing the bodies to be moved along these lines, allowing the vibration absorbing bodies to be located adjacent to the edge of a building.